FIG. 1 shows a longitudinal section of a known filter construction 100 used particularly in mobile phones in the 450-MHz frequency area. The construction comprises a low-loss printed circuit board 101 onto which finger-like projections 102 have been formed. Around each finger-like projection a cylindrical coil conductor, or helix, 103 has been wound to function as a resonator the electrical length of which is one quarter of the wavelength at the operating frequency. The lower ends, with respect to the position shown, of the helixes are grounded and the upper ends are open. The construction also includes a housing 104 that comprises outer walls and partition walls. Each helix is located inside a compartment of its own, separated from the next compartment by a partition wall. Partition walls may have holes of different sizes in various locations to realize electromagnetic coupling between adjacent helixes. In addition, inter-helix couplings can be realized through strip conductors 105 on the surface of the printed circuit board 101.
FIG. 2 shows a longitudinal section of a known ceramic filter construction 200 used especially in mobile phones in the 900-MHz frequency area. The filter construction is based on a block 201 made of a dielectric ceramic material such that the outer surface of the block is for the greater part plated with an electrically conductive coating 202 and has got holes 203 in it that wholly or partly extend through the ceramic block. Also the inner surfaces of the holes 203 are plated using an electrically conductive material. The inner coating of a hole is at one end in galvanic contact with the coating on the outer walls of the block so that the coating of the hole constitutes a .lambda./4 resonator in the same manner as the helix wire in the helix resonator described above. Coupling to the filter is realized through coupling strips 204 formed on the uncoated areas of the block 201. Electromagnetic coupling between resonators is realized through the ceramic material and it can be controlled by varying the amount and pattern of the coating on the block.
FIG. 3 shows a coaxial resonator construction 300 for frequencies above 2 GHz, known from the Finnish patent application FI-970525. For illustrative purposes, part of the electrically conductive casing 301 around the filter is cut out in the drawing. Partition walls 302 divide the casing into compartments in the same manner as in helix resonators. There is one coaxial resonator 303 in each compartment of the filter. FIG. 3 does not show the resonator in the middle compartment of the filter. In the lower parts of the partition walls 302 there are holes to realize electromagnetic couplings. The base plate 304 of the filter is a printed circuit board wherein electrically conductive areas of desired shape and size can be formed on both surfaces and all sides. On the top surface of the base plate there are conductive patterns 305 through which coupling to the resonators 303 is realized and which mediate in the electromagnetic coupling between resonators. On the bottom surface of the base plate there is a substantially continuous electrically conductive coating (not shown) that constitutes a ground plane and is in connection with the metal plating 306 on the edges of the base plate. There are gaps 307 in said metal plating that separate the continuous plating from ports strips 308 and 309. Port strips are narrow conductive areas at the edge of the printed circuit board that have connections to certain conductive patterns on the top surface of the printed circuit board and thereby to certain resonators. At each port strip there is a gap (not shown) in the electrically conductive coating on the bottom surface of the printed circuit board and on the side of the casing to prevent a short circuit between a port strip and the ground plane.
For the attachment of resonators 303 the printed circuit board 304 according to FIG. 3 has at each resonator a hole such that on the inner surface of the hole there is a metal plating or other electrically conductive coating connected to the electrically conductive coating, or ground plane, on the bottom surface of the printed circuit board. The inner surface of the hole need not be metal plated if electric coupling to the resonator can be made reliable enough in some other way. To ensure the best possible electric contact and to realize accurate electromagnetic dimensioning each hole may be encircled by a ring of conductive coating also on the top surface of the printed circuit board. The resonators can be soldered to their places or attached using electrically conductive glue, for example. In the filter according to FIG. 3 there is at the upper end of each resonator a bulge the function of which is to produce a so-called impedance step, i.e. impedance change point in the direction of the longitudinal axis of the resonator. The resonators may also be manufactured without said bulge.
The prior-art filter constructions described above have the problem that they are only applicable to frequencies ranging from a few hundred megahertz to a few gigahertz at the most. In communications systems utilizing new radio technology, such as the wireless local loop (WLL) and wireless local area network (WLAN), there is a visible tendency towards 10 to 20-GHz frequencies which, when realized, would render quarter-wave resonator constructions so small that their mass manufacture with sufficiently precise mechanical tolerances would be impossible, at least at a reasonable cost level.
For frequencies of tens of gigahertz and for optical frequencies filters have been manufactured using waveguides which usually are structures with a rectangular cross section wherein a dielectric core is surrounded by a coating which is reflective at the operating frequency. On both sides of a waveguide in the middle waveguides can be placed that have gaps at regular intervals in their coatings. With suitable location and dimensions of the gaps the waveguides will be coupled only at precisely predetermined frequencies so that the construction can be used as a filter. The manufacturing costs of such a construction are relatively high and repeatability in mass production is poor. In addition, the construction is rather big in size.